The present invention is directed generally to N,N′-dimethyl-meta-xylylenediamine (DM-MXDA) and to methods for producing DM-MXDA and related amines. The present invention also relates to amine compositions and amine-epoxy compositions containing DM-MXDA, methods of making amine-epoxy compositions, and articles, such as coatings, composites, and civil engineering products, produced from these amine-epoxy compositions.
Epoxy resins which are cured, hardened, or crosslinked with multifunctional amines, i.e., amine compounds having three or more active amine hydrogens, are well known in the industry. These materials are widely used in applications such as coatings, composites, and civil engineering applications, for example, formulations for flooring. In coating applications, some amine-cured epoxy formulations can be cured at room temperature to yield films with high mechanical strength, good water, chemical, and corrosion resistance, and excellent adhesion properties, particularly to metallic substrates. Thus, they are often employed as primers and topcoats for large structures such as ships, bridges, and industrial plants and equipment. Some amine-epoxy formulations provide excellent adhesion to concrete and other cementitious materials and, therefore, are often employed in sealers, primers, coatings, mortars, and grouts for concrete and other cementitious materials.
Before regulations placing limits on the volatile organic compound (VOC) content of amine-epoxy coatings, formulations were often based on solid epoxy resins. These resins are solid at room temperature. Coatings using solid epoxy resins usually dried very quickly, since only solvent evaporation, not chemical cure, was required for the coating to reach a dry-to-touch state.
Due to the VOC regulations, epoxy resins that are liquids at room temperature have replaced solid epoxy resins in many applications. This transition has resulted in several problems, for example, in coating applications. Amine-epoxy compositions based upon liquid epoxy resins tend to cure much more slowly than a comparable solid epoxy resin formulation, and this problem becomes more severe at lower temperatures. Shipyards, for example, often reside in locations with cold winters, and paint must be applied when temperatures are about 5° C. or colder. Many amine-epoxy coating formulations cure very slowly at these temperatures, often requiring at least 24 hours, and in some cases much more than 24 hours, to reach the “walk-on” dry state required so that painters can apply a second or third coat, if required. In the laboratory, the “walk-on” dry state is often estimated by the thumb-twist test method. Slow drying times can dramatically impact a shipyard's productivity. Thus, fast cure speed at below room temperature is a desirable property in many applications.
It is also beneficial to limit the volatility of the amine component in the amine-epoxy formulation. In addition to meeting VOC regulations, reducing volatility can reduce worker exposure and safety concerns.
Amine-epoxy coating formulations based on a liquid epoxy resin, as opposed to a solid epoxy resin, can also be less flexible than required for certain applications. For example, in ships employing modern double hull construction, the steel used in the two hulls that form the ballast tank is a thinner gauge than used in single hull ships. As a result of the thinner gauge, the steel flexes more which can lead to a stress crack failure of the coating, especially around welded joints. This in turn can lead to corrosion, which can be expensive to repair and can affect the ship's integrity. Further, in the rail car industry, there are also problems due to lack of coating flexibility at the weld seams. Additionally, coatings in many other applications require greater flexibility, for example, to achieve a desired impact resistance for a given application, or to post-form a metal after painting. In the end-use application, the amount of stress or deformation that the material undergoes, as well as the rate of deformation, are important factors for determining the flexibility required, and thus the suitability of a particular amine-epoxy composition or formulation. In civil engineering applications, for example, those involving concrete and other cementitious materials, amine-epoxy materials capable of withstanding greater expansion and contraction stresses, and capable of meeting crack bridging requirements, are also of interest.
Many epoxy coatings are over-coated with a second or third coating. The additional coatings are not limited to epoxy-based systems and can include other chemical coating systems (e.g., polyurethanes) in order to provide particular end-use properties, such as corrosion resistance, weatherability, etc. Intercoat adhesion in formulations based on liquid epoxy resins typically is less than comparable solid epoxy resin formulations, often leading to intercoat adhesion failures. Even when adequate intercoat adhesion for a liquid epoxy system is obtained, re-coating often must occur within a limited time frame if intercoat adhesion failures are to be avoided. This time is often referred to as the re-coat window.
Many amine-epoxy coatings suffer from problems referred to in the industry as blush, carbamation, and exudate. These problems, in part, are due to the incompatibility of the amine curing agent and the epoxy resin, which causes phase separation and results in amine migration to the coating surface. In primary amines, the migratory amine can react with CO2 present in the air, resulting in carbamation. Whether in the form of carbamation or the greasy surface layer referred to as exudate or blush, these surface defects detract from the appearance of the coating, and can lead to intercoat adhesion failures if the film is re-coated. These problems are generally worse for coatings applied and cured at colder temperatures, where amine-epoxy compatibility is reduced.
Often, epoxy coatings used in some of the aforementioned applications, such as coatings on metal, concrete, and cementitious substrates, require good chemical, water, and corrosion resistance. These barrier and weatherability properties of the coating can be important attributes to protect the substrate from environmental impact.
There are several broad classes of multifunctional amine curing agents that are employed in the amine-epoxy coating industry, including polyamides, Mannich bases (including phenalkamines), and amine adducts. None of these known products addresses the needs or solves the problems noted above. Accordingly, it is to this end that the present invention is directed.